At this point it seemed like we were out of the woods right up until I checked the fit of the helical gear to the mating feature on the new cutterhead and found the spline on the head wasn't even close to fitting the slot in the gear. Ugh.
I got in touch with contacts from both Holbren and Byrd and they confirmed that it wasn't as simple as receiving the wrong version head (they commented they are all the same) but that this planer seems to be different than everything they've come across in the past so they asked if I could take some measurements for comparison.
The slot in the helical gear measured out at about ~0.325" while the spline on the cutterhead measured ~0.400" exactly as spec'd by Byrd. This confirmed that the head was good but my planer was in fact different than 99% of the similar models out there.
After providing these measurements Garry from Byrd suggested that the easiest path forward would be to mill out the slot in the helical gear to match the cutterhead or if that wasn't something I could get done easily he offered to send me one of the standard gears with the hope that the only difference was the slot width. The offer was much appreciated but since I had to replace the gear shift handle anyway I ended up ordering a replacement gear through grizzly along with the other parts. If that doesn't fit then I'll have to fall back to milling out the slot in the original gear.
While I was reviewing the parts diagram (above) to find the right replacement parts I also noticed that my gearbox was clearly setup differently and after some head scratching it appears it only has gears to support a single speed and that the gear change handle was likely "seized" on purpose by bending the handle shaft to prevent someone from trying to shift into the non-existent 2nd speed. I've heard of planers like this with "single speed" gear boxes but I'd be surprised if this sort of hack was the typical production approach to dealing with the gear selector handle. At this point I'm guessing I will likely end up taking a similar approach once I get it put back together but I'll have to play around with the gearbox once I get the replacement parts to see if any better options are pop out. It was also interesting to note that my planer is missing the entire chain tensioner assembly (#106 etc. in the diagram below) nor the mounting features in the casting to support it.
Once I get parts and get this oddball back up and running I'll report back on how things worked out. Hopefully this info is useful to someone else out there that might have an odd-ball 15" 4 post planer like this one with a single speed gearbox, no chain tensioner, and non-standard cutterhead drive spline. It turns out these clones are not all the same after all...